The invention relates to a method for determining a starting gear ratio for a vehicle having a stepped variable speed transmission with several gear ratios.
Especially in the case of vehicles having multi-gear stepped variable speed transmissions, different gear ratios are suitable for starting of which one gear ratio is selected as the starting gear ratio according to load and driving mode.
In an unloaded industrial vehicle having a 16-gear transmission, for example, it can be advantageous for an optimum start on level roads to select the fifth gear as starting gear ratio, while in a fully loaded vehicle in an uphill road, it can be needed to start in the first or second gear. In a driving situation if too low a starting gear ratio is selected, this unnecessarily results in many gear changes. If the selected starting gear ratio is too high, the starting process lasts longer with the consequence of a higher load of the starting clutch or it has to be discontinued if the traction force available is not sufficient in the starting gear selected and started again.
When starting, the rotational speed difference, which in a stationary vehicle is between an engine-side and a transmission-side part of the starting clutch. During said slip phase, the starting clutch transmits a torque whereby the transmission input shaft and the vehicle are accelerated. During the slip phase, a considerable frictional work is produced in the starting clutch, customarily designed as dry clutch, whereby elevated temperatures are generated. At the end of the slip phase, the parts of the starting clutch rotate synchronously with the rotational speed of the engine. If too great a starting gear step is selected when starting, the acceleration of the transmission input shaft that can be achieved is only small so that the slip phase lasts much longer and the elevated temperatures generated can damage the starting clutch. On uphill gradient roads, there can occur the case in which the traction force is smaller than the gradient resistance whereby the vehicle starts to roll backwards.
While in vehicles, having conventional mechanical stepped variable speed transmissions, the driver chooses the starting gear step, methods for determining a starting gear step are preferably applied in automated stepped variable speed transmissions. It is possible to cause the suitable starting gear step be determined by an electronic control device and suggest it to the driver, e.g. via a display. Naturally, such a display can also be helpful for the driver in a vehicle having a conventional mechanical stepped variable speed transmission. It is also possible to let the calculated starting gear be automatically inserted by an automated stepped transmission or power shift transmission when reaching the parking state of the vehicle.
U.S. Pat. No. 5,406,862 discloses a method for determining a starting gear step in which the starting gear step is determined by an electronic control device according to the vehicle mass and the inclination of the road. In a characteristic field memory are stored the suitable starting gear steps according to the vehicle mass and the road inclination which is determined with a gradient sensor.
In different types or variants of vehicles, a specific transmission type is customarily used which can differ from each other in vehicle weight and in prime mover. The characteristic field in which the starting steps are stored has to be specifically covered for the variant, i.e. with different values for different vehicle variants.
The results of a specific value coverage of the characteristic field on the clutch load are not directly detectable in the process so that the results of a certain adjustment for the whole characteristic field have each to be expensively verified. Wrong adjustments can lead to destruction of the starting clutch.
Departing from the prior art, the problem of the invention is to develop for determining a starting gear a method which has a great reliability in relation to wrong adjustments, in which the adjustments are easily adaptable to different variants and which, taking into consideration the admissible load of the starting clutch, always makes the greatest possible starting gear step available.
The invention is solved by a method that includes the features of the main claim. Advantageous developments of the method are stated in the sub-claims.
According to the invention, the starting gear step is determined depending on an engine torque available for starting, a maximum slip time and/or a maximum frictional work of the starting clutch during the starting process. To this end, the available engine torque, a maximum admissible value for a slip time and/or a maximum admissible value for a frictional work of the starting clutch are first determined during the starting process. Beginning with a highest gear step suitable for starting, values for the slip time and/or the frictional work are then precalculated in a calculation loop according to the ratio of the gear step, the vehicle mass, the tractional resistance and an engine torque available for starting. The precalculated values are compared with the previously determined maximum admissible values. The calculation loop is repeated with the next smaller gear step suitable for starting until a gear step is found in which the precalculated values are smaller than or equal to the maximum admissible values. The gear step is issued as starting gear step. The tractional resistance is cyclically calculated during the travel in short time intervals so that after parking the vehicle an actual starting gear can be made available.
The respective optimum starting step is at the same time directly determined depending on the maximum loading capacity fo the starting clutch. The maximum loading capacity of the starting clutch is in no case exceeded. For adaptation to different vehicle variants, there are essentially vehicle parameters to be changed, such as the engine torque available for starting and tractional resistance coefficients which are usually known.
The maximum admissible values for the slip time and/or the frictional work are advantageously stored in the electronic control devicexe2x80x94preferably in a characteristic field memoryxe2x80x94according to road inclination, vehicle mass and/or load state of the clutch. Hereby is obtained a great reliability relative to wrong adjustments which can cause damage to the starting clutch. The engine torque available for starting can likewise be determined according to road inclination and/or vehicle mass; not in all driving situations need they be started under full engine load.
The vehicle mass can be delivered by the driver or also measured with sensors in the running gear. But it is especially advantageously determined using commonly existing wheel rotational speed sensors by a method where, during two measurements temporarily offset within a measuring period, a traction parameter and a motion parameter resulting therefrom are determined.
Such a method has been disclosed, e.g. in EP 0 666 435 A2 which comprises two temporarily offset measurements of the traction force produced by the prime mover and the acceleration resulting therefrom. It is to be assumed that the unknown tractional resistance in two consecutive measurements is substantially equal for both measurements so that the unknown parameters can be shortened. In this known method, both during a traction phase and during a traction-free phase, while one clutch is open for the purpose of gear change of a stepped variable speed transmission, a value of a wheel torque and a value of acceleration of the vehicle are respectively determined and from them can be calculated the actual mass of the vehicle.
It has proved especially advantageous when in a development of the method the first of the two measurements comprises a first data collection period, the second of the two measurements a second data collection period, the duration of both data collection period is longer than a minimum duration, the traction force parameter corresponds to the time integral of the traction force acting during the respective data collection period and the motion parameter corresponds to the speed change of the vehicle occurring during the respective data collection period. If this method is made, the basis for determining the mass, the starting gear step can be determined without an additional sensor being needed therefor.
The method for determining the mass is based on the following equation:       M    Fzg    =                              ∫          t0          t1                ⁢                              F            Zug                    ⁢                      xe2x80x83                    ⁢                      ⅆ            t                              -                        M          Gang                ⁡                  (                      v1            -            v0                    )                            (              v1        -        v0        -        v3        +        v2            )      
wherein:
At this point reference is had to the applicant""s non-published older patent application DE 198 37 380.5 whose object is a method for determining mass. Its content is explained as pertinent to the disclosure content of the instant application.
In another advantageous development of the inventive method for determining a starting gear step, the parameter dependent on driving mode which comprises the ascending resistance is determined from the vehicle mass, an actual traction force and an actual vehicle acceleration resulting therefrom while the vehicle moves. Compared to the known method, hereby is spared a gradient sensor to determine the inclination of the road.
Another advantageous development of the method provides that the parameter dependent on driving mode comprise, together with the uphill resistance of the vehicle, the rolling resistance of the vehicle which also acts during the starting process. Hereby is better reproduced the positive tractional resistance which is mainly composed of the uphill resistance and the rolling resistance of the vehicle. The air resistance can be disregarded in the starting process.
From the parameters vehicle mass MFzg, actual traction force FZug and vehicle acceleration xcex1Fzg can be calculated, by solving the motion equation, the traction resistance which is equal to the sum of the rolling and uphill resistances.